Abstract: In the past few years, sleeve gastrectomy has rapidly emerged as the most common weight loss procedure in the United States, displacing the gastric bypass procedure. However, there is shockingly little evidence of the procedure's long-term outcomes. There are growing concerns about the long-term safety and durability of the procedure. The best existing evidence on sleeve gastrectomy is from small randomized trials that have several limitations, including: limited follow up (e.g. 3 years); lack of statistical power to examine important secondary outcomes (e.g., reoperations and revisions); exclusion of important patient subgroups (e.g., older, disabled); and a lack of consideration of economic outcomes (e.g., health care use and expenditures). Because insurance coverage for sleeve gastrectomy proceeded in a staggered, regional approach, there is a unique opportunity to use these policy changes as an instrumental variable in comparing sleeve gastrectomy vs. laparoscopic gastric bypass, the gold-standard bariatric procedure. We have the following Specific Aims: Aim 1. To compare the long-term safety of sleeve gastrectomy vs. gastric bypass. We will use two complementary claims datasets, including a large, commercially insured population from MarketScan and the national fee-for-service Medicare population, to evaluate the long-term safety of the sleeve gastrectomy compared to laparoscopic gastric bypass surgery up to 7 years after their procedure. Aim 2. To evaluate long-term health care use and expenditures of sleeve gastrectomy vs. gastric bypass. Using data from MarketScan and Medicare, we will evaluate the use of health care services (hospital days, emergency department visits, and outpatient visits) and health care expenditures for sleeve gastrectomy (vs. gastric bypass) for up to 7 years after their procedure. Aim 3. To evaluate heterogeneity in safety, health care use, and expenditures for sleeve gastrectomy vs. gastric bypass. We will evaluate safety, health care use, and expenditures, for important patient subgroups, including older and disabled populations (e.g., Medicare beneficiaries), those with poor baseline health status (e.g., high comorbid disease burdens and high baseline expenditures), and select comorbid diseases (e.g., patients with type 2 diabetes) and for providers with different characteristics (e.g., surgeons with more vs. less volume). A better understanding of the long-term outcomes of sleeve gastrectomy (vs. gastric bypass) will allow patients to make more informed decisions regarding long-term outcomes and need for additional surgery. These data will also be invaluable for informing private insurers and the Center for Medicare and Medicaid Services (CMS) as they reassess their coverage decisions for bariatric surgery.